A.K.H. JABERI*+, J.A.H. AL SAEDI*+, MARIA MERNEA*#, L. PETRESCU*, O. CĂLBOREAN*, I. VASILE**, D.F. MIHĂILESCU*
* Department of Anatomy, Animal Physiology and Biophysics, Faculty of Biology, University of Bucharest, Bucharest, Romania, +Authors with equal contribution
** Department of Computational Physics and Information Technologies, “Horia Hulubei” National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), Măgurele, Romania
Gram-negative bacteria lipopolysaccharides (LPS) are lipids with a chemical structure comprising a lipidic region with several hydrocarbonate chains and a large saccharide region. Due to their structure, LPS can associate through hydrophobic forces, but also can dissolve in water. Here we investigated the association of LPS molecules on the surface of a silicon crystal by terahertz (THz) spectroscopy. We measured the THz absorption of LPS molecules directly deposited on the crystal or deposited on the crystal coated with a CaCl2 solution. Resulting spectra present a non-linear increase of THz absorption, with two absorption shoulders at 1.05 THz (~35 cm–1) and 1.37 THz (~45 cm–1). The presence of the CaCl2 solution does not influence the shoulders localization, suggesting a similar association of LPS molecules in both situations. Based on the comparison with previous THz studies on phospholipid bilayers, we suggested a layer ordering of LPS molecules in our samples that was validated through molecular modelling methods. We modelled LPS monolayers and simulated the THz spectra by normal modes analysis (NMA) and molecular dynamics (MD). The NMA spectrum of a LPS monolayer resulted in agreement with experiments, allowing us to validate the proposed LPS layer organization and the spectra simulation method.
Key words: lipopolysaccharides, THz spectroscopy, spectra simulation.
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